The present invention relates to a stretch film (cling film) for packaging food and other items and a method for manufacturing the same.
The stretch wrap film material utilized to prepackage various kinds of food such as fruit and vegetables, fresh fish, meat, delicatessen, etc. by stretch wrapping the item to be packaged either directly or after placing the item on a light weight tray made of plastic, etc. used to be produced mainly from poly(vinyl chloride). Poly(vinyl chloride) film, however, would pose problems associated with environmental pollution and sanitary and health hazards. It is under said circumstance that polyolefin resins such as polyethylene, ethylene 1-butene copolymer, ethylene vinyl acetate copolymer have come to be used as raw materials for the stretch wrap film in these years.
Nonetheless, problems that ensued from the diversion from poly(vinyl chloride) to polyolefin were such that films produced from polyethylene and ethylene 1-butene copolymer are relatively hard, and hence not readily stretched and that ethylene vinyl acetate copolymer film is susceptible to breakage after the wrapping operation. Accordingly, there has been proposed a stretch wrap film material obtained by laminating a ethylene vinyl acetate copolymer layer onto both sides of a 1-butene-based resin layer (Japanese Laid-open Patent Application No. 1986 (Sho 61)-89040). Although the aforesaid problem associated with relatively high hardness and low stretchability of the film was thereby resolved, there still remained unresolved such further problem that the film gives only a narrow latitude of compatibility with the automatic wrapping machine. Another drawback was that sagging tends to occur when the item to be wrapped was loaded on the film.
For the purpose of remedying said shortcomings, there was proposed a film produced from a resin obtained by mixing ethylene (meth)acrylic acid (meth) acrylic ester copolymer with ethylene (meth)acrylic acid copolymer (Japanese Laid-open Patent Application No. 1994 (Hei 6)-322192). Nevertheless, said film did not give a satisfactory stress at elongation, etc., and hence, there still remained a need for such film that will have a further satisfactory compatibility with the automatic wrapping machine. Moreover, said film had the demerit of affording only a low productivity inherent to the blown (inflation) film extrusion system by which said film can be obtained.
The object of the present invention is to provide a polyolefin stretch film which possesses (a) a large elongation, cut property of traverse direction, adherence/conformance property (cling property), elastic recovery after distention by fingertips, etc., which are essential properties required of the stretch wrap film material to be used on the automatic wrapping machine, (b) a good balance between the stress at 100% elongation in the longitudinal direction and the stress at 100% elongation in the transverse direction, (c) the ability to accomplish wrinkle-free packaging, and (d) an excellent productivity; and to provide a process for manufacturing such film.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
According to the present invention, the polyolefin stretch film of the present invention is produced from a resin composition comprising a polyolefin resin either consisting entirely of ethylene (meth)acrylic acid copolymer or ethylene (meth)acrylic acid (meth)acrylic ester terpolymer or consisting of a mixture of both, as a base resin and being, insofar as necessary, compounded with an anti-fogging agent and a tackifier, which is characterized in that the stress at 100% elongation in the longitudinal direction (the machine direction) is 12-30 MPa and the stress at 100% elongation in the transverse direction (the cross direction) is 5-11 MPa.
A preferred embodiment of the present invention is a polyolefin stretch film produced from a resin composition comprising 100 parts by weight of said polyolefin resin, 0.5-12 parts by weight of a tackifier and 1-5 parts by weight of an anti-fogging agent.
The preferred polyolefin stretch film of the present invention comprises a film produced from said resin composition by the extrusion film forming technique employing a straight manifold die. Furthermore, the preferred polyolefin stretch film of the present invention also comprises a film produced from said resin composition by the extrusion film forming technique employing a straight manifold die which has undergone an orientation preventing treatment.
The process for manufacturing the polyolefin stretch film of the present invention comprises carrying out an orientation preventing treatment in the course of producing a film by the extrusion film forming technique employing a straight manifold die from a resin composition comprising a polyolefin resin either consisting entirely of ethylene (meth)acrylic acid copolymer or ethylene (meth)acrylic acid (meth)acrylic ester terpolymer or consisting of a mixture of both, as a base resin and being, insofar as necessary, compounded with an anti-fogging agent and a tackifier either during the film forming step or after the film has been formed.
Referring to the polyolefin stretch film of the present invention, a detailed description is given in the following as for the polyolefin resin composition constituting such film, the process for manufacturing the film and the stretch film thereby obtained.
The resin composition from which the film was produced of the present invention is comprising a polyolef in resin either consisting entirely of ethylene (meth)acrylic acid copolymer or ethylene (meth)acrylic acid (meth)acrylic ester terpolymer or consisting of a mixture of both, as a base resin and being, insofar as necessary, compounded with an anti-fogging agent and a tackifier.
The ethylene (meth)acrylic acid copolymer is manufactured by copolymerizing ethylene with methacrylic acid or copolymerizing ethylene with acrylic acid by the well-known radical polymerization process, etc., and the ratio of the structural unit component (hereinafter referred to simply as xe2x80x9ccomponentxe2x80x9d) derived from (meth)acrylic acid is preferably 3-20 mole %. When the ratio of (meth)acrylic acid component declines to an excessively low level deviating from said range, the flexibility and the elastic recovery after distention by fingertips of the obtained film tend to prove insufficient. On the contrary, when the ratio of (meth)acrylic acid component exceeds said range, the cut property of traverse direction of the obtained film often turns out to be insufficient.
The melt flow rate of ethylene (meth)acrylic acid copolymer (as determined in accordance with ASTM D1238 at a temperature of 190xc2x0 C.) is generally 0.5-30 g/10 min.
The ethylene (meth)acrylic acid (meth)acrylic ester terpolymer is obtained by either copolymerizing ethylene with acrylic acid and (meth)acrylic ester or copolymerizing ethylene with acrylic acid and (meth)acrylic ester by the well-known radical polymerization process, etc.
The (meth)acrylic ester in this case is represented by acrylic ester or methacrylic ester whose alcohol residue is a hydrocarbon having 1-8 carbon atoms. As examples of such (meth)acrylic ester, there can be cited methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, isobutyl methacrylate, etc. Among the above-exemplified ester, the preferred ones are acrylic ester or methacrylic ester whose alcohol residue is a hydrocarbon having 4-6 carbon atoms.
A preferred ethylene (meth)acrylic acid (meth)acrylic ester terpolymer contains 3-20 mole % or preferably 5-15 mole % of a component derived from (meth)acrylic acid and 0.3-20 mole %, or preferably 0.7-15 mole %, or more preferably 1.0-15 mole % of a component derived from (meth)acrylic ester. In particular, an ethylene (meth)acrylic acid (meth)acrylic ester terpolymer containing 2 or less mole %, or preferably 0.3-2 mole % of a component derived from (meth)acrylic ester is preferred, in as much as it imparts an excellent tensile strength, and hence is by far less susceptible to breakage during the wrapping operation.
The melt flow rate of ethylene (meth)acrylic acid (meth)acrylic ester terpolymer (as determined in accordance with ASTM D1238 at a temperature of 190xc2x0 C.) is generally 0.5-30 g/10 min.
It is preferable that 50-100 parts, or more preferably 60-100 parts by weight of ethylene (meth)acrylic acid (meth)acrylic ester terpolymer is contained in 100 parts by weight of the polyolefin resin component in the resin composition constituting the stretch film of the present invention. A film having an excellent tensile strength and a good balance between the stress at 100% elongation in the longitudinal direction and the stress at 100% elongation in the transverse direction can be obtained as long as the resin composition remains in said range, particularly in the range of a low (meth)acrylic ester component content, or more particularly at 7% by weight or less of (meth)acrylic ester component content in the polyolefin resin.
It is furthermore preferable that the polyolefin resin substantially comprises ethylene (meth)acrylic acid (meth)acrylic ester terpolymer.
Said polyolefin resin either consisting entirely of ethylene (meth)acrylic acid copolymer or ethylene (meth)acrylic acid (meth)acrylic ester terpolymer, or consisting of a mixture of both constitutes the base resin for the polyolefin stretch film of the present invention, and by way of using it as the base resin, there can be achieved the cut property of traverse direction, adherence/conformance property and elastic recovery after distention by fingertips, which are properties essentially required of the stretch wrap film material, while such base resin exhibits an excellent processibility in the extrusion film forming system employing a straight manifold die.
For the purpose of the present invention, other resin(s) may be added to said base resin to an extent not detrimental to the object of the present invention. As examples of such other resin(s), there can be cited an individual copolymer or a mixture of 2 or more kinds of copolymer selected from among polyethylene, ethylene-xcex1-olefin copolymer, ethylene vinyl acetate copolymer, etc.
For the purpose of the present invention, it is preferably that the ratio of (meth)acrylic ester component to said base resin is 7% by weight or less, or more preferably 5% by weight or less, in as much as the tensile strength of the film remains high within such range. Although there is no particular lower limit for the (meth)acrylic ester component content of the base resin, it is generally 0.1% by weight or more, or preferably 1% by weight. or more, or more preferably 2% by weight or more.
For the purpose of improving the adherence/conformance property and clarity of the stretch film, it is preferable that a resin composition be prepared by adding 0.5-12 parts by weight, or preferably 2-10 parts by weight of a tackifier, and 1-5 parts by weight, or preferably 1.5-4 parts by weight of an anti-fogging agent to 100 parts by weight of a polyolef in resin consisting either entirely of said ethylene (meth)acrylic acid copolymer or ethylene (meth)acrylic acid (meth)acrylic ester terpolymer, or consisting of a mixture of both.
The tackifier is a solid amorphous polymer and is of such type that is generally used in adhesive tapes, paint, hot melt adhesive, and for similar applications. According to the type of starting monomer to be polymerized, there can be cited the following resins as examples of such tackifier.
Aliphatic hydrocarbon resin principally produced from C4 fraction, C5 fraction, and a mixture thereof obtained by cracking petroleum, naphtha, etc., or optional fraction(s) thereof, for example, isoprene, 1,3-pentadiene, etc. derived from C5 fraction; aromatic hydrocarbon resin produced from a styrene derivative(s) and indenes derived from C9 fraction obtained by cracking petroleum, naphtha, etc.; aliphatic-aromatic copolymerized hydrocarbon resin obtained by copolymerizing optional fractions selected from C4 fraction and C5 fraction with C9 fraction; alicyclic hydrocarbon resin obtained by hydrogenating an aromatic hydrocarbon resin; synthetic terpene hydrocarbon resin having a structure containing aliphatic, alicyclic and aromatic hydrocarbon resins; and terpene hydrocarbon resin produced from xcex1xcex2-pinene in turpentine oil; cumarone-indene hydrocarbon resin produced from indenes and styrenes in coal tar-based naphtha; low molecular weight styrene resin; rosin-based hydrocarbon resin, etc.
Among said tackifiers, aliphatic hydrocarbon resin and alicyclic hydrocarbon resin obtained by hydrogenating aromatic hydrocarbon resin are preferably used, inasmuch as they possess favorable miscibility with ethylene (meth)acrylic acid copolymer. Furthermore, it is preferable to use alicyclic hydrocarbon having a softening point (as determined by a ring-and-ball test) of 105-150xc2x0 C., or particularly 110-140xc2x0 C., and a ratio of hydrogenation to the aromatic nucleus is 80% or more, or particularly 85% or more.
The anti-fogging agent is an additive compounded with the resin composition for the purpose of preventing the moisture in the air from fogging the film after it is condensed on the surface of the film, so long as the additive is capable of rendering the film surface hydrophilic and thus spreading water drops produced by condensation of the moisture, such additive will sufficiently serve as an anti-fogging agent without any limitation.
For example, surface-active agents may be used as such anti-fogging agent. There can be cited as specific examples sorbitan fatty acid ester such as sorbitan monooleate, sorbitan monolaurate, sorbitan monobehenate, sorbitan monostearate, etc.; glycerin fatty acid ester such as glycerin monooleate, glycerin monostearate, etc.; polyglycerin fatty acid ester such as diglycerin monooleate, diglycerin sesquilaurate, diglycerin sesquioleate, tetraglycerin monooleate, tetraglycerin monostearate, hexaglycerin monooleate, decaglycerin monooleate, decaglycerin monolaurate, etc.; polyoxialkylene ether such as polyoxiethylene lauryl ether, etc.; aliphatic amine such as lauryl diethanol amine; fatty acid amide such as oleamide, etc. The above-identified compounds may be used either individually or as a mixture of two or more kinds.
The stretch film of the present invention may include other additive(s) to an extent not detrimental to the object of this invention.
It is preferable that the stretch film of the present invention is manufactured from said resin composition by the extrusion film forming technique employing a straight manifold die. In the course of the film extruding operation employing a straight manifold die, it is preferable that the operation be carried out at a forming temperature of 200-250xc2x0 C., at a draft (drawdown) ratio (the stretching ratio within the draft (drawdown) distance) of 60 or less, at a draft (drawdown) distance between the tip of the die and the point where the film contacts the chilled roll of 50 mm or longer, while the film is taken off at a film temperature of 40xc2x0 C. or more and a stretching ratio of 1.2 times or less as the means to prevent the film from being oriented. It is preferable to carry out annealing as a further means for preventing orientation of the film, for instance, at a film temperature of 50-90xc2x0 C. so as to limit the dimensional change of the film in the longitudinal direction (the shrinkage ratio) within a range of 1-5%. The orientation preventing treatment may be carried out either by carrying out the former film forming method during the film forming step, or by carrying out the latter method which comprises the annealing treatment. Furthermore, an orientation preventing treatment may be carried out by a combination of the said two methods. Alternatively, the annealing treatment may be carried out after the film is formed.
The stretch film of the present invention thus formed has a thickness of 10-200 xcexcm, a stress at 100% elongation in the longitudinal direction of 12-30 MPa, or preferably 16-30 MPa, or more preferably 18-25 MPa, or particularly preferably 18-24 MPa, a stress at 100% elongation in the transverse direction of 5-11 MPa, or preferably 6-10 MPa, or more preferably 6-8 MPa. In consequence of the oppression of orientation in the longitudinal direction of the film, there can be achieved a good balance between the stress at 100% elongation in the longitudinal direction and the stress at 100% elongation in the transverse direction, and the ratio of [the stress at 100% elongation in the longitudinal direction] to [the stress at 100% elongation in the transverse direction] (LD/TD) settles preferably in a range of 1.5-3.0, or more preferably 1.7-2.5.
It is by the aforesaid procedure that there can be obtained a stretch film having excellent cut property of traverse direction, adherence/conformance property, and elastic recovery after distension by fingertips which is less susceptible to wrinkle formation in the package operation and has an excellent compatibility with the automatic packaging machine, thus permitting continuous stretch packaging operation interrupted only by least frequent film breakage.
Additionally, inasmuch as the stretch film of the present invention can be manufactured by the extrusion film forming technique employing a straight manifold die which affords a high productivity without going through the route of the conventional blown (inflation) film extrusion system, it is preferred for the economical reason, too.
The stretch film of the present invention may be finished, insofar as necessary, by laminating on one side or both sides of the film layer composed of said resin composition a film layer or film layers composed of other resin(s). In such case, the stresses at 100% elongation as a laminated film are within said range.
As an example of such other resin to constitute the surface layer in the laminated film, there can be cited polyolefins mainly comprising ethylene or propylene. As specific examples thereof, there can be cited at least one kind of polymer which can be formed into film such as polyethylene, polypropylene, ethylene-xcex1-olefin copolymer, propylene-ethylene-butene copolymer, etc. Melt flow rates (as determined in accordance with ASTM D1238 at a temperature of 190xc2x0 C.) of those polymers generally fall within a range of 0.5-30 g/10 min.
In case the stretch film of the present invention is a laminated film, the total thickness of other resin layers constituting surface layer(s) is generally 1-50 xcexcm, or preferably 2-30 xcexcm.
As the method for forming the laminated film, there may be employed well-known techniques such as the coextrusion method and the extrusion lamination method, both of which employing a straight manifold die.